The present invention relates to a process for cracking an olefin-rich hydrocarbon feedstock which is selective towards propylene in the effluent. In particular, olefinic feedstocks from refineries or petrochemical plants can be converted selectively so as to redistribute the olefin content of the feedstock in the resultant effluent thereby to provide a recoverable propylene content.
It is known in the art to use zeolites to convert long chain paraffins into lighter products, for example in the catalytic dewaxing of petroleum feedstocks. While it is not the objective of dewaxing, at least parts of the paraffinic hydrocarbons are converted into olefins. It is known in such processes to use crystalline silicates for example of the MFI type, the three-letter designation xe2x80x9cMFIxe2x80x9d representing a particular crystalline silicate structure type as established by the Structure Commission of the International Zeolite Association. Examples of a crystalline silicate of the MFI type are the synthetic zeolite ZSM-5 and silicalite and other MFI type crystalline silicates are known in the art. GB-A-1323710 discloses a dewaxing process for the removal of straight-chain paraffins and slightly branched-chain paraffins, from hydrocarbon feedstocks utilising a crystalline silicate catalyst, in particular ZSM-5. U.S. Pat. No. 4,247,388 also discloses a method of catalytic hydrodewaxing of petroleum and synthetic hydrocarbon feedstocks using a crystalline silicate of the ZSM-5 type. Similar dewaxing processes are disclosed in U.S. Pat. No. 4,284,529 and U.S. Pat. No. 5,614,079. The catalysts are crystalline alumino-silicates and the above-identified prior art documents disclose the use of a wide range of Si/Al ratios and differing reaction conditions for the disclosed dewaxing processes.
GB-A-2185753 discloses the dewaxing of hydrocarbon feedstocks using a silicalite catalyst. U.S. Pat. No. 4,394,251 discloses hydrocarbon conversion with a crystalline silicate particle having an aluminium-containing outer shell.
It is also known in the art to effect selective conversion of hydrocarbon feeds containing straight-chain and/or slightly branched-chain hydrocarbons, in particular paraffins, into a lower molecular weight product mixture containing a significant amount of olefins. The conversion is effected by contacting the feed with a crystalline silicate known as silicalite, as disclosed in GB-A-2075045, U.S. Pat No. 4,401,555 and U.S. Pat. No. 4,309,276. Silicalite is disclosed in U.S. Pat. No. 4,061,724.
Silicalite catalysts exist having varying silicon/aluminium atomic ratios and different crystalline forms. EP-A-0146524 and 0146525 in the name of Cosden Technology, Inc. disclose crystalline silicas of the silicalite type having monoclinic symmetry and a process for their preparation. These silicates have a silicon to aluminium atomic ratio of greater than 80.
WO-A-97/04871 discloses the treatment of a medium pore zeolite with steam followed by treatment with an acidic solution for improving the butene selectivity of the zeolite in catalytic cracking.
A paper entitled xe2x80x9cDe-alumination of HZSM-5 zeolites: Effect of steaming on acidity and aromatization activityxe2x80x9d, de Lucas et al, Applied Catalysis A: General 154 1997 221-240, published by Elsevier Science B. V. discloses the conversion of acetone/n-butanol mixtures to hydrocarbons over such dealuminated zeolites.
It is yet further known, for example from U.S. Pat. No. 4,171,257, to dewax petroleum distillates using a crystalline silicate catalyst such as ZSM-5 to produce a light olefin fraction, for example a C3 to C4 olefin fraction. Typically, the reactor temperature reaches around 500xc2x0 C. and the reactor employs a low hydrocarbon partial pressure which favours the conversion of the petroleum distillates into propylene. Dewaxing cracks paraffinic chains leading to a decrease in the viscosity of the feedstock distillates, but also yields a minor production of olefins from the cracked paraffins.
EP-A-0305720 discloses the production of gaseous olefins by catalytic conversion of hydrocarbons. EP-B-0347003 discloses process, for the conversion of a hydrocarbonaceous feedstock into light olefins. WO-A-90/11338 discloses a process for the conversion of C2-C12 paraffinic hydrocarbons to petrochemical feedstocks, in particular to C2 to C4 olefins. U.S. Pat. No. 5,043,522 and EP-A-0395345 disclose the production of olefins from paraffins having four or more carbon atoms. EP-A-0511013 discloses the production of olefins from hydrocarbons using a steam activated catalyst containing phosphorous and H-ZSM-5. U.S. Pat. No. 4,810,356 discloses a process for the treatment of gas oils by dewaxing over a silicalite catalyst. GB-A-2156845 discloses the production of isobutylene from propylene or a mixture of hydrocarbons containing propylene. GB-A-2159833 discloses the production of isobutylene by the catalytic cracking of light distillates.
It is known in the art that for the crystalline silicates exemplified above, long chain olefins tend to crack at a much higher rate than the corresponding long chain paraffins.
It is further known that, when crystalline silicates are employed as catalysts for the conversion of paraffins into olefins, such conversion is not stable against time. The conversion rate decreases as the time on stream increases, which is due to formation of coke (carbon) which is deposited on the catalyst.
These known processes are employed to crack heavy paraffinic molecules into lighter molecules. However, when it is desired to produce propylene, not only are the yields low but also the stability of the crystalline silicate catalyst is low. For example, in an FCC unit a typical propylene output is 3.5 wt %. The propylene output may be increased to up to about 7-8 wt % propylene from the FCC unit by introducing the known ZSM-5 catalyst into the FCC unit to xe2x80x9csqueezexe2x80x9d out more propylene from the incoming hydrocarbon feedstock being cracked. Not only is this increase in yield quite small, but also the ZSM-5 catalyst has low stability in the FCC unit.
There is an increasing demand for propylene in particular for the manufacture of polypropylene.
The petrochemical industry is presently facing a major squeeze in propylene availability as a result of the growth in propylene derivatives, especially polypropylene. Traditional methods to increase propylene production are not entirely satisfactory. For example, additional naphtha steam cracking units which produce about twice as much ethylene as propylene are an expensive way to yield propylene since the feedstock is valuable and the capital investment is very high. Naphtha is in competition as a feedstock for steam crackers because it is a base for the production of gasoline in the refinery. Propane dehydrogenation gives a high yield of propylene but the feedstock (propane) is only cost effective during limited periods of the year, making the process expensive and limiting the production of propylene. Propylene is obtained from FCC units but at a relatively low yield and increasing the yield has proven to be expensive and limited. Yet another route known as metathesis or disproportionation enables the production of propylene from ethylene and butene. Often, combined with a steam cracker, this technology is expensive since it uses ethylene as a feedstock which is at least as valuable as propylene.
EP-A-0109059 discloses a process for converting olefins having 4 to 12 carbon atoms into propylene. The olefins are contacted with an alumino-silicate having a crystalline and zeolite structure (e.g. ZSM-5 or ZSM-11) and having a SiO2/Al2O3 molar ratio equal to or lower than 300. The specification requires high space velocities of greater than 50 kg/h per kg of pure zeolite in order to achieve high propylene yield. The specification also states that generally the higher the space velocity the lower the SiO2/Al2O3 molar ratio (called the Z ratio). This specification only exemplifies olefin conversion processes over short periods (e.g. a few hours) and does not address the problem of ensuring that the catalyst is stable over longer periods (e.g. at least a few days) which are required in commercial production. Moreover, the requirement for high space velocities is undesirable for commercial implementation of the olefin conversion process.
Thus there is a need for a high yield propylene production method which can readily be integrated into a refinery or petrochemical plant, taking advantage of feedstocks that are less valuable for the market place (having few alternatives on the market).
EP-A-0921179, EP-A-0921177 and EP-A-0921176 all in the name of Fina Research S.A. disclose processes for the production of propylene by catalytic cracking of an olefin-containing feedstock. The feedstock contains olefins of C4 or greater. Although the propylene yield is high, there is a need to improve the yield yet further, particularly over long cycle times of the catalyst.
It is also disclosed in EP-A-0921179 and EP-A-0921177 that in order to provide a stable catalyst, when the hydrocarbon feedstock contains one or more dienes, together with the olefins, the dienes are hydrogenated prior to the catalytic cracking process. Typically, the hydrogenation is performed over a palladium-based catalyst. The reason that dienes are removed prior to the catalytic cracking process is that dienes tend to form coke precursors for the crystalline silicate catalysts. When coke is deposited on the catalyst, the catalysts are gradually deactivated. The applicant has found that even if the hydrocarbon feed has been hydrotreated by employing a hydrogenation process upstream of the catalytic cracking process, dienes are always detected at the outlet of the catalytic cracking reactor. Thus there is a need for an improved method for removing the presence of dienes or coke precursors in the catalytic cracking process, thereby to achieve reduced deactivation of the catalyst. Moreover, it would be desirable to avoid having to perform a separate hydrogenation step upstream of the catalytic cracking step.
On the other hand, crystalline silicates of the MFI type are also well known catalysts for the oligomerisation of olefins. For example, EP-A-0031675 discloses the conversion of olefin-containing mixtures to gasoline over a catalyst such as ZSM-5. As will be apparent to a person skilled in the art, the operating conditions for the oligomerisation reaction differ significantly from those used for cracking. Typically, in the oligomerisation reactor the temperature does not exceed around 400xc2x0 C. and a high pressure favours the oligomerisation reactions.
GB-A-2156844 discloses a process for the isomerisation of olefins over silicalite as a catalyst. U.S. Pat. No. 4,579,989 discloses the conversion of olefins to higher molecular weight hydrocarbons over a silicalite catalyst. U.S. Pat. No. 4,746,762 discloses the upgrading of light olefins to produce hydrocarbons rich in C5+ liquids over a crystalline silicate catalyst. U.S. Pat. No. 5,004,852 discloses a two-stage process for conversion of olefins to high octane gasoline wherein in the first stage olefins are oligomerised to C5+ olefins. U.S. Pat. No. 5,171,331 discloses a process for the production of gasoline comprising oligomerising a C2-C6 olefin containing feedstock over an intermediate pore size siliceous crystalline molecular sieve catalyst such as silicalite, halogen stabilised silicalite or a zeolite. U.S. Pat. No. 4,414,423 discloses a multistep process for preparing high-boiling hydrocarbons from normally gaseous hydrocarbons, the first step comprising feeding normally gaseous olefins over an intermediate pore size siliceous crystalline molecular sieve catalyst. U.S. Pat. No. 4,417,088 discloses the dimerising and trimerising of high carbon olefins over silicalite. U.S. Pat. No. 4,417,086 discloses an oligomerisation process for olefins over silicalite. GB-A-2106131 and GB-A-2106132 disclose the oligomerisation of olefins over catalysts such as zeolite or silicalite to produce high boiling hydrocarbons. GB-A-2106533 discloses the oligomerisation of gaseous olefins over zeolite or silicalite.
DE-A-3708332 discloses a process for the thermal conversion of ethylene, in a mixture with naphtha, in a steam cracker.
It is an object of the present invention to provide a process for using the less valuable olefins present in refinery and petrochemical plants as a feedstock for a process which, in contrast to the prior art processes referred to above, catalytically converts olefins into lighter olefins, and in particular propylene.
It is another object of the invention to provide a process for producing propylene having a high propylene yield and purity.
It is a further object of the present invention to provide such a process which can produce olefin effluents which are within, at least, a chemical grade quality.
It is yet a further object of the present invention to provide a process for producing olefins having a stable olefinic conversion and a stable product distribution over time.
It is yet a further object of the present invention to provide a process for converting olefinic feedstocks having a high yield on an olefin basis towards propylene, irrespective of the origin and composition of the olefinic feedstock.
The present invention provides a process for cracking an olefin-rich hydrocarbon feedstock which is selective towards propylene in the effluent, the process comprising contacting a hydrocarbon feedstock containing one or more olefinic components of C4 or greater with a crystalline silicate catalyst to produce an effluent having a second composition of one or more olefinic components of C3 or greater, the feedstock and the effluent having substantially the same olefin content by weight therein characterised in that ethylene is added to the feedstock before the feedstock contacts the catalyst.
Preferably, at least a part of the ethylene is recycled from the effluent.
More preferably, the ethylene comprises from 0.1 to 50 wt % of the hydrocarbon feedstock.
The process may further comprise adding to the feedstock hydrogen gas at a hydrogen partial pressure of up to 15 bar. The hydrogen partial pressure can be varied depending on the composition of the feedstock, the LHSV and the nature of the catalyst. The hydrogen partial pressure is preferably up to 15 bar, more preferably up to 7.5 bar, yet more preferably from 0.1 to 7.5 bar and most preferably from 0.1 to 5 bar. In order to retain the olefinicity of the hydrocarbons i.e. to ensure that the feedstock and effluent have substantially the same olefin content, the hydrogen partial pressure is typically up to 7.5 bar when the olefin partial pressure and the LHSV are kept within readily implementable ranges (i.e. olefinic partial pressure of from 0.1 to 2 bar and LHSV of from 10 to 30 hxe2x88x921) with the preferred catalysts of the invention. However, although the olefinicity tends to decrease with increasing hydrogen partial pressure as a result of hydrogenation of the olefins to form paraffins, a hydrogen partial pressure of up to 15 bar can be employed by utilising different olefin partial pressures, LHSV""s and catalysts.
The hydrogen may be pure or impure hydrogen and freshly introduced into the catalytic cracker or recycled from another step or process.
In one preferred arrangement, both the ethylene and the hydrogen are together recycled as a common stream, back into the feedstock from the effluent.
The present invention can thus provide a process wherein olefin-rich hydrocarbon streams (products) from refinery and petrochemical plants are selectively cracked not only into light olefins, but particularly into propylene. The olefin-rich feedstock may be passed over a crystalline silicate catalyst with a particular Si/Al atomic ratio of at least 180 for example by synthesis or obtained after a steaming/de-alumination treatment. The feedstock may be passed over the catalyst at a temperature ranging between 500 to 600xc2x0 C., an olefin partial pressure of from 0.1 to 2 bars and an LHSV of from 10 to 30 hxe2x88x921 to yield at least 30 to 50% propylene based on the olefin content in the feedstock.
The present invention is predicated on the discovery by the present inventors that the addition of ethylene, either pure or impure, fresh or recycled, to the feedstock, optionally together with C5 and/or C6 olefins, can increase the yield of propylene in the selective catalytic cracking of an olefin-containing feedstock containing primarily C4 olefins. Ethylene may be introduced together with the hydrogen feed. When ethylene is added to the feedstock in this way, around 20% of the ethylene is converted into other olefins with a propylene selectivity of typically at least around 20%. The amount of ethylene added may vary from about 0.1 to about 50 wt % based on the weight of the remaining constituents of the feedstock.
The ethylene may be introduced together with an additional feed of C5 and/or C6 olefins. This in turn increases the propylene yield. Such a combined feed avoids the requirement to separate recycled ethylene from hydrogen and methane and also gives an overall propylene yield of around 30 to 50% on an olefin basis.
The inventors have also found, with respect to a preferred aspect of the invention, that dienes are always detected at the outlet of the catalytic cracking reactor even if the feed had been hydrotreated before the catalytic cracking step in an attempt to hydrogenate dienes to form olefins. The present inventors concluded that dienes may be accordingly formed in the catalytic cracking reactor as a result of degradation of the olefins. Thus without being bound by theory it is believed by the present inventors that the addition of hydrogen to the feedstock enhances the stability of the catalyst by reducing coke precursor formation by reducing the formation of dienes and/or by reducing the dehydrogenation of dienes into coke precursors.
Accordingly, the inventors have found that the addition of hydrogen to the olefin-containing feedstock should limit the formation of dienes, and in turn should limit any catalyst deactivation. The addition of hydrogen to the feedstock is believed (without being bound by theory) to tend to drive the reaction to form dienes in the opposite direction thereby altering the thermodynamic equilibrium of the degradation of the olefins. In this way, reduced presence of dienes in the catalytic cracker tends to reduce the formation of coke on the catalyst, and thus increases the stability of the catalyst. The inventors have also found that C4 dienes tend to be less detrimental to as regards coke formation than C5 or C6 dienes.
This addition of hydrogen avoids the requirement for selective hydrogenation of the dienes upstream of the catalytic cracking process. This also tends to increase the cycle time for any given catalyst i.e. the time between successive catalyst regenerations.
In this specification, the term xe2x80x9csilicon/aluminium atomic ratioxe2x80x9d is intended to mean the Si/Al atomic ratio of the overall material, which may be determined by chemical analysis. In particular, for crystalline silicate materials, the stated Si/Al ratios apply not just to the Si/Al framework of the crystalline silicate but rather to the whole material.
The silicon/aluminium atomic ratio is preferably greater than about 180. Even at silicon/aluminum atomic ratios less than about 180, the yield of light olefins, in particular propylene, as a result of the catalytic cracking of the olefin-rich feedstock may be greater than in the prior art processes. The feedstock may be fed either undiluted or diluted with an inert gas such as nitrogen. In the latter case, the absolute pressure of the feedstock constitutes the partial pressure of the hydrocarbon feedstock in the inert gas, and in the ethylene, and in the hydrogen when present.